(1) Field of the Invention
The present invention relates to a sealing composition for sealing ceramics mutually or for sealing ceramics with metal, and a sealing method. More particularly, the present invention relates to a sealing compositon which is used in the sealing of arc tubes, which are made of translucent alumina ceramics and used for high-pressure vapor discharge lamps, with ceramics or refractory metal, and a sealing method using the sealing composition.
(2) Description of the Prior Art
There have hitherto been developed and used various sealing compositions for sealing translucent alumina tubes, whic are used as arc tubes for high-pressure vapor discharge lamps, with alumina ceramics or refractory metals, such as niobium, molybdenum and the like.
The sealing compositions of this kind must have a high bonding strength and at the same time have heat resistance and resistance against the high temperature vapor of alkali metal during lamp operation. As such sealing compositions, there have been used titanium-nickel alloys, called the activated metal process, and glass compositions such as aluminum oxide-calcium oxide-magnesium oxide systems (hereinafter, abbreviated as Al.sub.2 O.sub.3 --CaO--MgO systems), aluminum oxide-calcium oxide-silicon dioxide-magnesium oxide-barium oxide systems (hereinafter, abbreviated as Al.sub.2 O.sub.3 -CaO--SiO.sub.2 --MgO--BaO systems), or aluminum oxide-calcium oxide-silicon dioxide systems (hereinafter, abbreviated as Al.sub.2 O.sub.3 --CaO--SiO.sub.2 systems). However, titanium-nickel alloy is insufficient in heat resistance, and when the sealed portion is heated to high temperature, the gas tightness of the sealed portion is likely to deteriorate. The glass compositions based on the system of Al.sub.2 O.sub.3 --CaO--SiO.sub.2 are apt to crystallize at high temperature, causing cracks in the sealed portion. Further, since these glass compositions are poor in corrosion resistance against alkali metal vapor at high temperature, there is a danger of leakage of the sealed portion.